1. Field of the Invention
The present invention relates to a method and device thereof for detecting broken word lines in a memory array, and more particularly, to a method and device thereof for detecting broken word lines in a Dynamic Random Access Memory (DRAM) array.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a DRAM array 100. As shown in FIG. 1, the DRAM array 100 comprises word lines WL1, WL2, and WL3, bit lines BL1, BL2, and BL3, switches SW11, SW12, SW21, SW22, SW31, and SW32, and memory cells M11, M12, M21, M22, M31, and M32. When a user writes a datum to a memory cell, the voltage on the corresponding word line is raised to a high voltage VH for turning on the switch corresponding to the memory cell. For example, when the user writes a datum “1” to the memory cell M22, the voltage on the word line WL2 is raised to the voltage VH so as to turn on the switch SW22, which allows the connection between the memory cell M22 and the bit line BL2, and consequently the datum “1” is written to the memory cell M22. When no data is written to a memory cell, the voltage on the corresponding word line remains at a low voltage VL for turning off the switch corresponding to the memory cell. As shown in FIG. 1, when a user writes a datum to the memory cell M22, only the voltage on the word line WL2 is raised to the high voltage VH, which only turns on the switches SW21 and SW22, while the word lines WL1 and WL3 corresponding to the remaining memory cells not accessed stays at the low voltage VL.
During the fabrication process of a DRAM, particle impurities exist. The particles result in adjacent word lines being short-circuited so that when the word line is charged for writing data to the corresponding memory cell, the voltage on the charged word line cannot reach to the predetermined voltage VH since the charge on the charged word line is dispersed over the other word lines short-circuited to the charged word line. As shown in FIG. 1, the particle P exists between the word lines WL1 and WL2, causing a short circuit between the word lines WL1 and WL2. Thus, when a user writes data to the memory cells corresponding to the word lines WL1 or WL2, the short circuit caused by the particle P fails the writing action and the data cannot be written correctly into the memory cells corresponding to the word lines WL1 or WL2. For example, when a user writes a datum to the memory cell M22, the voltage on the word line WL2 should be raised to the high voltage VH and remain at the high voltage VH throughout the entire writing action, but because of the short circuit between the word lines WL1 and WL2 caused by the particle P, which allows current leakage to flow from the word line WL2 to the word line WL1, the voltage on the word line WL2 cannot stay at the high voltage VH throughout the entire writing action, and instead gradually falls from the high voltage VH.
Please refer to FIG. 2. FIG. 2 is a diagram illustrating the voltage on the word line WL2. As shown in FIG. 2, in an ideal situation (the particle P does not exist), the voltage on the word line WL2 is raised to the high voltage VH for a period of time T1 (as shown by the dashed line). In fact, the voltage on the word line WL2 is gradually lowered by the word line WL1 since the voltage on the word line WL1 is at the low voltage VL at the same time because of the effect of the particle P. When the voltage on the word line WL2 drops to the threshold voltage VE, if the writing action is not finished, the switch SW22 is turned off and the data cannot be written to the memory cell M22, thus the writing action fails. As shown in FIG. 2, when the word line WL2 is activated (raised to the high voltage VH), if the writing action is not finished by the period T2, the voltage on the word line WL2 drops below the threshold voltage VE, which turns off the switches SW21 and SW22. Thus, the data stored in the memory cells M21 and M22 corresponding to the word line WL2 are possibly incorrect. Consequently, the user may read incorrect data from those memory cells, which causes great inconvenience.